The absorption rates of calcium (Ca) and inorganic phosphate (P) will be determined in the human small intestines by intestinal perfusion. The effect of the luminal concentration of these ions, the luminal pH and anatomical site on the absorption rates will be studied. The hormones, calcitonin, parathyroid hormone and the active metabolites of vitamin D3, are known to influence mineral metabolism. The effects of these hormones on Ca and P absorption will be studied in normal subjects and in selected diseases. Patients deficient in one of the three hormones, e.g., hypoparathyroidism, and those with a pathological excess, e.g., medullary carcinoma of the thyroid gland, will be included in these studies. The simultaneous analysis of luminal fluid for the concentration of cyclic nucleotides by radioimmunoassay may identify a relationship between the absorption of water and ions and these nucleotides. The proposed research may clarify and enlarge our knowledge of the hormonal control of Ca and P absorption in the human small intestines.